jcomm sot-4 april 2007 revisiting the xbt fall rate equation (sippican deep blue):implications and...
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JCOMM SOT-4 April 2007
Revisiting the XBT Fall Rate Equation (Sippican Deep Blue):Implications and Recommendations
JCOMM/SOT-4Derrick Snowden
Gustavo GoniMolly Baringer
JCOMM SOT-4 April 2007
Outline
• Introduction to the data• XBT Error Partioning• Estimating a New Fall Rate
Equation• Implications for Climate Studies• Conclusions and
Recommendations
JCOMM SOT-4 April 2007
Instruments ListSystem Name
Launcher Type
Recorder Type
Controller Software (PC)
aomlauto AOML Autolauncher
MK-12 AOML (DOS 386 PC)
devilhand Sippican LM-3A
Devil Devil Software(Win XP PC)
seasauto AOML Autolauncher
MK-21 SEAS2000 (WinXP PC)
seashand Sippican LM-3A
MK-21 SEAS2000(Win XP PC)
sioauto SIOAutolauncher
MK-21 SEAS2000(Win XP PC)
siok98 SIO Autolauncher
MK-12
366 total profiles
JCOMM SOT-4 April 2007
XBT/CTD Profiles
JCOMM SOT-4 April 2007
Outline
• Introduction to the data• XBT Error Partioning• Estimating a New Fall Rate
Equation• Implications for Climate Studies• Conclusions and
Recommendations
JCOMM SOT-4 April 2007
Partitioning XBT Error
XBT differs from the CTD due to :A: inaccurate depth measurementsB: inaccurate temperature measurements
A B
JCOMM SOT-4 April 2007
Fall Rate Equation
a: Change of mass of the probe due to spooling wire.
b: Probe geometry and initial terminal velocity
c: Describes startup transients i.e. initial conditions
Currently used FRE (WMO 1770:52) for Deep Blue probes contains no constant term ( c ).
JCOMM SOT-4 April 2007
Fall Rate Error: Initial Conditions
Hallock and Teague 1992
JCOMM SOT-4 April 2007
Outline
• Introduction to the data• XBT Error Partioning• Estimating a New Fall Rate
Equation• Implications for Climate Studies• Conclusions and
Recommendations
JCOMM SOT-4 April 2007
Determining Depth Offset
Focus on the temperature gradient: derivative of temperature with respect to depth
Minimize temperature error effect
JCOMM SOT-4 April 2007
Depth Offset Prior to Correction
JCOMM SOT-4 April 2007
Depth Correction Algorithm
• Assemble the depth offsets (dZ)• Recalculate XBT depths
Znew = Zxbt+dZ
• Back calculate time (t)• Fit new fall rate equation to (t) z = at^2 + bt + c
JCOMM SOT-4 April 2007
Depth Offset: Post-Correction
JCOMM SOT-4 April 2007
Summary of Fall Rate Coefficients
A B C
Wmocode 52
-0.00225 6.691 0
this study -0.00191 6.487 0
this study -0.00299 6.639 -4.5117
other studies (1)
-0.00131 – -0.00329
6.440 – 6.798
0 - 0
1: Taken from Hanawa et al 1995
JCOMM SOT-4 April 2007
Outline
• Introduction to the data• XBT Error Partioning• Estimating a New Fall Rate Equation• Implications for Climate Studies
(Wijffels and Thresher 2007)• Conclusions and Recommendations
JCOMM SOT-4 April 2007
Basin Averaged Offset
JCOMM SOT-4 April 2007
Global Trends
JCOMM SOT-4 April 2007
Recommendations•Future Work: Tropical Atlantic Cruise (May 2007) will conduct additional side by side XBT-CTD comparisons•Reinvorgorate research toward understanding fall rate issues
•Metadata to include coefficients and the form of the equations (it isn’t sufficient to list A,B need to know z=at^2 + bt + c + … (including exchange files and metadata servers)
•Quality flags should be listed level by level not just for the entire profile
•Communication with the manufacturers to better understand the timing and nature of changes in XBT manufacturing procedures.
JCOMM SOT-4 April 2007
Implications for Climate Studies
A B